The present invention relates to a process for recovery of nickel from spent catalyst. The present invention also relates to a process for the recovery of nickel from spent catalyst wherein alumina is also recovered as an important by-product. The present invention particularly relates to a process for recovery of nickel and alumina from spent catalyst by direct leaching with sulphuric acid in presence of small amount of an additive. The invention is useful for recovery of both nickel and sources of nickel and therefore important from the view of environmental protection, resource recycling and conservation.
The consumption of nickel in India is about 20,000 MTPY (metric tonnes per year which is entirely imported. Though several by-products/wastes such as nickel sludge generated during the pickling of stainless steel, grinding waste of AlNiCo magnets, and spent catalyst from the fertiliser, petrochemical and hydrogenation plants are available, at present none of such sources are being exploited commercially. Therefore, to meet the ever-growing demand, the effort has been made to provide an innovative and cost effective process for recovery of both nickel and alumina from the nickel spent catalyst of the fertiliser plants, besides producing alumina as a by-product thereby generating minimal waste for disposal.
Nickel based catalysts are the catalysts of choice in several industries due to their low cost competing substitutes. Such catalysts use alumina and silica as supports. Deactivated nickel catalysts are not considered to be regeneratable by ordinary techniques and also pose a significant waste disposal problem. Hence, it is necessary to develop new extraction techniques for processing of such spent catalyst to obtain pure metal for re-reuse.
Considerable efforts have been made to recover nickel from spent catalyst by hydrometallurgical processes. In most of the earlier work the spent catalyst had to undergo pre-treatment step to processing (Inooka Masayoshi, Japan, Kokai, Yokyo, Koho 7811621, 11 Oct. 1978; Telly, George L., U.S. Pat. No. 4,721,600, 20 Jan. 1988; Giurea et al Rom Ro 85578, 29 Sep. 1984). In general chlorination (Gravey, G., LeGroff J. and Gonin C., Jan. 8, 1980, U.S. Pat. No. 4,182,747), pressure leaching with ammonium hydroxide-ammonium carbonate or sodium hydroxide (Gutnikov G. Mar. 2, 1971, U.S. Pat. No. 3,567,433; Millsap W. A. and Reisler N., 1978, Eng. and Min J., Vol. 179 (5), p. 105. ) and sodium carbonate roasting (Castange H., Gravey G. and Roth A, Feb. 21, 1978, U.S. Pat. No. 4,075,277 were applied. After a pre-treatment spent catalyst is directly is leached with water/acid/alkali. Reduction roasting followed by sulphuric acid leaching of a spent catalyst from hydrogenation plant to produce nickel oxide reported only 83% of overall recovery (P. Alex, T. K. Mukherjee and M. Sundaresan, 1991, Metals Materials and processes, Vol. 3(2), P.-81). Roasting followed by selective chlorination at 400xc2x0 C. of spent catalyst under Cl2+air, Cl2+N2 and Cl2+N2 was investigated and maximum recovery of only 80% was reported (Gaballah I. and Dona M., 1993, the Paul E. Queneau Int. Symp. on Extractive Metallurgy of Copper, Nickel and Cobalt, Vol. 1, p. 1253, Ed. R. G. Reddy and R. N. Weizenbach, minerals, Metals and Material Society). Direct leaching of a spent catalyst has several disadvantages such as requirement of high strength acid as high as 20-30% and poor nickel dissolution of other metal ions which are the major problems for the subsequent processing steps of final product recovery. Neutralisation of highly acidic leach liquor requires high amount of alkali and will generate huge quantity of waste cake which will add cost to the process, besides creating environmental problem.
Thought several attempts have been made to recover nickel by various processes involving pre-treatment such as roasting, reduction/alkali/chlorination roasting etc. followed by acid/alkali/neutral leaching, however, there are rarely any attempt made to accelerate the metal dissolution process by adding additive such a persulphate salts of ammonium, sodium, potassium, etc.
The main object of the present invention is to provide a process for extraction of nickel from spent catalyst which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a leaching process for extraction of nickel from spent nickel catalyst in presence of little amount of catalytic established processes and avoids requirement of high strength acid for selective and quantitative dissolution of nickel from this resource.
Still another objective of the present invention is to develop a suitable process for the recovery of alumina from spent catalyst as a valuable by-product suitable for special grade refractories or for reuse as supports for catalyst.
Accordingly, the present invention provides a process for extraction of nickel from spent nickel catalyst which comprises
i) adding a persulphate based additive along with fine sized nickel catalyst in the sulphuric acid solution and stirring by a magnetic needle/glass stirrer and maintaining the solid liquid ratio in the range of xc2xd-{fraction (1/10)} (wt./vol.),
ii) keeping the temperature of the slurry obtained in step(i) in the range of 40 to 100xc2x0 C. for a period of 0.5 to 6 h,
iii) allowing the slurry to settle and then filtering the slurry to obtain leach liquor containing nickel and alumina as solid residue,
iv) washing the solid residue to remove entrapped liquor and drying at 110-120xc2x0 C. to get a by-product containing high alumina,
v) purifying the said leach liquor by precipitating iron and other impurities using lime and filtering to obtain pure nickel sulphate solution,
vi) crystallising or precipitating the leach liquors to obtain nickel sulphate crystal or nickel hydroxide,
vii) reducing nickel hydroxide to obtain nickel metal powder or nickel oxide.
In an embodiment of the present invention the spent nickel catalyst used is selected from nickel catalyst having particle size in the range of xe2x88x92211 to +38 xcexcm and has a composition in range:
Ni: 5-20%, Fe 0.1-1%, Al2O3: 70-90% SO2: 0-6%
In another embodiment of the present invention the spent nickel catalyst used is selected from persulphate salts of sodium, potassium and ammonium and has concentration in the range of 0.25-4% (w/w).
In still another embodiment of the present invention the sulphuric acid is of commercial grade and has concentration in the range of 2-12% (v/v). This acid concentration is obtained after mixing with the wash solution generated from the leaching step earlier and is used for further leaching.
In still another embodiment of the present invention the pulp density of leaching is in the rage of 10-100%. The higher pulp density leaching generates concentrated leach solution requiring low capital investment and energy.
In the process of present invention the leach slurry is filtered and the residue is washed with very dilute sulphuric acid solution. The wash liquor containing 10-30 g/L Ni is recycled for the leaching of the fresh spent catalyst is in oxide phase and dissolution in sulphuric acid occurs as:
NiO+H2SO4xe2x80x94NiSO4+H2Oxe2x80x83xe2x80x83(1)
The iron free leach liquor is evaporated crystallize nickel as nickel sulphate. The purified leaching liquor can also be precipitated as nickel hydroxide and nickel metal from this can be obtained by known method called hydrogen/carbothermic reduction process. Alternatively nickel metal powder can be produced by the known process by aqueous hydrogen reduction of purified leach liquor.
Novelty of the present invention is the use of a catalytic additive which has not been used earlier for direct leaching of nickel catalyst to recover nickel and alumina simultaneously. Another feature of the invention is complete nickel dissolution (99.9%) from the spent catalyst without any prior treatment such as roasting, reduction/alkali/chlorination roasting etc. which are necessary steps in earlier development. The complete dissolution of nickel in the leaching stage produces very bright coloured alumina as a valuable by-product, which may find applications as high alumina refractoriness and a support for catalyst.